Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic or anodic electrodeposition. However, in 1972, cationic (or cathodic) electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. For example, throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
Multilayered coating composites for metal substrates, for example, substrates used in the appliance and automobile industries, typically have involved electrodeposition coatings as an initial resinous coating layer to protect the metal substrate from corrosion. However, two-coat application by the electrodeposition process is known in the art. For example U.S. Pat. Nos. 4,988,420; 4,840,715; and 5,275,707 disclose multi-layered composite coatings applied by electrodeposition wherein electroconductive pigments are included in a first electrodeposited acrylic resinous coating, and subsequently a second coating is electrodeposited over the conductive first coating. Typically, these second electrodeposition coatings are applied for durability and decorative purposes.
The term “blank” refers to a flat or substantially flat section cut or “sheared” from a coiled metal strip and subsequently formed into a part, such as front and side panels for appliances, e.g., refrigerators, washers and dryers, metal office furniture, e.g., filing cabinets and desks, and building products, e.g., fluorescent lighting fixtures. Often holes must be punched in the blanks.
Coated metal blanks can offer many advantages in the manufacture of such products. Coated blanks can be “stacked” for storage in a vertical stacker while awaiting subsequent coating, forming, fastening and/or assembly processes. This can result in a reduction of inventory storage space as well as a reduction in in-process inventory. Also, primed-only blanks which have been cut to specification for various end-use products can be stacked off-line awaiting subsequent top coating steps. A wide variety of top coating compositions (e.g., liquid coatings and powder coatings) can be applied to the primed blanks using various application techniques, for example, electrodeposition, spray or roll coating techniques. In this way, a wide variety of colors can be delivered in a relatively short time.
As mentioned above, blanks can be cut from pre-coated or pre-painted coiled metal substrates or, alternatively, from coiled metal stock prior to coating the coiled metal. Problems can arise when blanks are cut from pre-coated metal. The shearing of these blanks (thus creating blanks having “sheared edges”) and the hole punching process typically creates exposed sheared ends and edges (i.e., edges devoid of protective coating), thus necessitating application of additional corrosion inhibitors to these areas. Special corrosion protection is especially critical if the finished product is subjected to high humidity conditions or aggressive detergents. Moreover, the coil-applied coating must meet strict flexibility requirements in order to withstand the shearing and punching processes without fracturing and/or losing adhesion at the edges of the sheared/punched area, as well as post-forming processes.
For the above-stated reasons, blanks cut from uncoated coiled metal stock, which are subsequently coated and formed into parts, can offer several advantages. First, sheared ends and edges are coated during the overall coating process, thus eliminating the additional step of applying a corrosion inhibitor to these edges. Further, although the coatings applied to pre-sheared blanks must meet the flexibility requirements necessary for withstanding post-forming processes, the need for coatings capable of withstanding the harsh shearing and punching processes is eliminated.
U.S. Pat. No. 5,439,704 teaches a combined coil and blank powder coating line which has the capability of coating coiled metal strips to form pre-coated metal coil stock as well as coating pre-sheared and/or punched blanks with the same coating line. Due to the necessity of placing the blanks on a horizontal support surface for transport, however, only the topside of the blank can be powder coated, thus leaving the underside uncoated.
U.S. Pat. No. 5,908,667 discloses a process for producing a multilayer lacquer coating of low dry film thickness in which a primer of an electrodepositable aqueous coating composition is electrophoretically applied onto (presumably both sides of) a conductive substrate and subsequently cured to form an electrically conductive primer on the substrate. A base coat is formed thereover by electrodeposition of a color-giving and/or effect-producing aqueous electrodepositable coating composition. The multilayer coating process is particularly useful for coating automobiles or pre-formed automobile parts.
It would be desirable to provide a post-formable, multi-layer composite coating on a metal blank using, at least in part, an electrodeposition process. The process would advantageously include electrodeposition of a corrosion inhibitive conductive primer to the blank surfaces followed by electrodeposition of an appearance enhancing top coat, or, alternatively, application of a non-electrophoretic top coating. Such a process would provide the aforementioned desired coating properties with efficient paint utilization and fast cure times.